Apparatuses for achieving high pressures have been known for over a half century. Typical ultrahigh pressure apparatuses include piston-cylinder presses, cubic presses, tetrahedral presses, belt presses, girdle presses, and the like. Several of these apparatuses are capable of achieving ultrahigh pressures from about 4 GPa to about 7 GPa.
High pressure apparatuses are commonly used to synthesize diamond and cubic boron nitride (cBN). Generally, source materials and other raw materials can be selected and assembled into a high pressure assembly which is then placed in the high pressure apparatus. Under high pressure, and typically high temperature, the raw materials combine to form the desired product. More specifically, graphite, non-diamond carbon or even diamond can be used as a source material in diamond synthesis, while hexagonal boron nitride (hBN) can be used in cBN synthesis. The raw material can then be mixed or contacted with a catalyst material. Diamond synthesis catalysts such as Fe, Ni, Co, and alloys thereof are commonly used. Alkalis, alkaline earth metals, or compounds of these materials can be used as the catalyst material in cBN synthesis. The raw material and catalyst material can then be placed in a high pressure apparatus wherein the pressure is raised to an ultrahigh pressure, e.g., 5.5 GPa. An electrical current can then be passed through either a graphite heating tube or graphite directly. This resistive heating of the catalyst material is sufficient to cause melting of the catalyst material, e.g., typically about 1300° C. for diamond synthesis and about 1500° C. for cBN synthesis. Under such conditions, the source material can dissolve into the catalyst and then precipitate out in a crystalline form as either diamond or cBN.
Typically, either an isothermal method or temperature gradient method is used to synthesize diamond. Each method takes advantage of the solubility of carbon under various conditions, e.g., temperature, pressure, and concentrations of materials. The isothermal method involves use of a carbon source material, metal catalyst, and sometimes a diamond seed. The carbon source is most frequently graphite or other forms of carbon material. Under high pressures and high temperatures, graphite is much more soluble in molten catalyst than diamond. Therefore, graphite tends to dissolve or disperse into the molten catalyst, or create a colloidal suspension therewith, up to the saturation point. Excess carbon can then precipitate out as diamond. Typically, a diamond seed can be surrounded by a thin envelope of molten catalyst, e.g., Fe, Ni, Co, and their alloys. In this case, the carbon can dissolve into and diffuse across the molten catalyst envelope toward the diamond seed of a diamond nucleus. Due to the presence of a thin molten catalyst layer, this type of isothermal process is also often referred to as a thin film process.
In contrast, the temperature gradient method involves maintaining a temperature gradient between a carbon source and the diamond seeds which are separated by a relatively thick layer of molten catalyst. The carbon source is kept at a relatively higher temperature than the diamond seed. As such, the carbon is more soluble in the hotter regions. The carbon then diffuses toward the cooler region where the diamond seed is located. The solubility of carbon is reduced in the cooler regions, thus allowing carbon to precipitate as diamond at the diamond seed. Typically, the molten catalyst layer is relatively thick in order to maintain a sufficient temperature gradient, e.g., 20° C. to 50° C., and is therefore also often referred to as a thick film process.
Unfortunately, currently known high pressure crystal synthesis methods have several drawbacks which limit their ability to produce large, high-quality crystals. For example, isothermal processes are generally limited to production of smaller crystals useful as superabrasives in cutting, abrading, and polishing applications. Temperature gradient processes can be used to produce larger diamonds; however, production capacity and quality are limited. Several methods have attempted to overcome these limitations. Some methods incorporate multiple diamond seeds; however, a temperature gradient among the seeds prevents achieving optimal growth conditions at more than one seed. Some methods involve providing two or more temperature gradient reaction assemblies such as those described in U.S. Pat. No. 4,632,817. Unfortunately, high quality diamond is typically produced only in the lower portions of these reaction assemblies. Some of these methods involve adjusting the temperature gradient to compensate for some of these limitations. However, such methods involve additional expense and variables in order to control growth rates and diamond quality simultaneously over different temperatures and growth materials.
Therefore, apparatuses and methods which overcome the above difficulties would be a significant advancement in the area of high pressure crystal growth.